• silent_list is directly picklable as is; no need to do anything about
  it. While we're at it, also get rid of__str__ --__str__ is
  documented to fall back on__repr__, so the latter is sufficient.
• When pickling GridSpec and SubplotSpec, set _layoutbox to None, so
  that no special loading is needed.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way